Re: raid6 check/repair

On Tuesday November 27, davidsen@tmr.com wrote:

Thiemo Nagel wrote:

quoted

Dear Neil,

thank you very much for your detailed answer.

Neil Brown wrote:

quoted

While it is possible to use the RAID6 P+Q information to deduce which
data block is wrong if it is known that either 0 or 1 datablocks is 
wrong, it is *not* possible to deduce which block or blocks are wrong
if it is possible that more than 1 data block is wrong.

If I'm not mistaken, this is only partly correct.  Using P+Q redundancy,
it *is* possible, to distinguish three cases:
a) exactly zero bad blocks
b) exactly one bad block
c) more than one bad block

Of course, it is only possible to recover from b), but one *can* tell,
whether the situation is a) or b) or c) and act accordingly.

I was waiting for a response before saying "me too," but that's exactly 
the case, there is a class of failures other than power failure or total 
device failure which result in just the "one identifiable bad sector" 
result. Given that the data needs to be read to realize that it is bad, 
why not go the extra inch and fix it properly instead of redoing the p+q 
which just makes the problem invisible rather than fixing it.

Obviously this is a subset of all the things which can go wrong, but I 
suspect it's a sizable subset.

Why do think that it is a sizable subset.  Disk drives have internal
checksum which are designed to prevent corrupted data being returned.

If the data is getting corrupt on some buss between the CPU and the
media, then I suspect that your problem is big enough that RAID cannot
meaningfully solve it, and "New hardware plus possibly restore from
backup" would be the only credible option.

NeilBrown